1. Field of the Invention
This invention relates to a laser-beam machining method and a laser-beam machining device for carrying out machining to pierce a cutting target such as a steel sheet.
2. Description of the Prior Art
In a laser-beam machining method of cutting a machining target such as a steel sheet by irradiating a laser beam outputted from a laser oscillator to the machining target in a converged state through a nozzle having a small hole, piercing work or boring work is required at the starting time of machining.
A serious problem faced by the piercing is the presence of scattered dross. Since boring of a machining target such as steel sheet or the like is carried out by melting the machining target by laser beam while jetting assist gas through the nozzle, scattering of dross resulting from melting the machining target by the laser beam occurs due to the assist gas jetted through the nozzle. There is a danger that the scattered dross may be adhered to the nozzle to damage the nozzle or be adhered to an operator to burn the operator. Further, the scattered dross adhered to the surface of the machining target hinders the subsequent machining for the machining target.
For preventing a nozzle from being damaged due to the scattered dross, there are known methods such as a method of making a nozzle of a metal of a high melting point material and that of placing a nozzle largely apart from a machining target only at the time of piercing (See Japanese Patent Applications Laid-open Nos. 63-52792 and 4-200891).
In addition, for the purpose of ensuring the safety of an operator, there are known methods such as a method of covering a machining point to prevent the dross from being scattered at the time of piercing and that of controlling the scattering direction of dross (See Japanese Patent Applications Laid-open Nos. 9-136177, 10-137970, 6-142972 and 10-225787).
However, according to the above methods, the scattered dross is adhered to the surface of the machining target, resulting in the hindrance of the subsequent cutting. In this connection, for the purpose of preventing the scattered dross from adhering to the surface of the machining target, there is disclosed a method of shielding a machining point to collect the dross in Japanese Patent Application Laid-open No. 9-271980. However, this method is supposed to be used in association with piercing based on pulse output which is determined in advance in terms of such size and quantity of scattered dross as will enable the scattered dross to be absorbed and collected (See Japanese Patent Application Laid-open No. 5-185261).
Recently, the improvement on a laser oscillator for higher power has been made, and such improvement brings about a tendency toward increasing of the thickness of a steel sheet serving as a machining target. When such a steel sheet of a large thickness is subjected to piercing based on pulse output as described above, there is a problem in that it takes a long time to carry out the piercing. On the contrary, if piercing is carried out with laser power of continuous output, as in the case of normal cutting, and also at its maximum level, the piercing can be completed quickly. However, in this case, a large quantity of dross will be produced disadvantageously, resulting in the occurrence of adhesion of such dross in the shape of a lump to the surface of the machining target. On the other hand, if piercing is carried out with laser power of low continuous output, it will take a long time to carry out the piercing and adhesion will occur disadvantageously.
An object of the present invention is to provide a laser-beam machining method, a laser-beam machining device and an auxiliary tool for piercing which permit piercing at high speed.
Another object of the present invention is to provide a laser-beam machining method, a laser-beam machining device and an auxiliary tool for piercing which may prevent dross from being scattered, resulting in no occurrence of adhesion of the dross to a machining target.
According to the present invention, a nozzle having a hole diameter (i.e., a nozzle having a hole diameter of not more than 2.5 mm) larger than a converged laser beam diameter and smaller than a hole diameter of a cutting nozzle is used as a piercing nozzle. Further, the piercing is carried out on condition that an underlay formed by a metal sheet having a hole in the center is placed on the machining target, so that the underlay is permitted to catch the scattered dross so as to prevent the dross from adhering to the surface of the machining target.
Further, the piercing nozzle is connected to the tip of the cutting nozzle at the time of piercing, for the reason that the piercing carried out using a cutting nozzle having a relatively small hole diameter may shorten the time taken for the piercing and may cause less scattering of the dross, in comparison with the piercing carried out using a cutting nozzle having a relatively large hole diameter. In addition, the piercing nozzle is mounted to the cutting nozzle detachably or is mounted to be movable between a piercing position and a shunting position.
A clearance for cooling down the scattered dross is provided between the piercing nozzle and the underlay. There are also provided a blade driven in the clearance to remove the scattered dross from the surface of the underlay and a scattered dross removing means to remove the scattered dross by flowing gas or a fluid into the clearance.
Further, the piercing nozzle added to the cutting nozzle and the underlay are provided in a pair as an auxiliary tool for piercing.